Retroreflective materials which employ cube-corner type reflecting elements are now well-known in the art. Cube-corner reflecting elements are trihedral structures which have three mutually perpendicular lateral faces meeting at a single corner, such as that configuration defined by the corner of a room or a cube. The retroreflectivity typically achieved by cube-corner type reflecting elements is through the principle of total internal reflection. A transparent cube-corner element receives a ray of incident light at an angle and sends it back in the same direction. See, for example, U.S. Pat. Nos. 3,924,929, 4,672,089, 4,349,598, and 4,588,258.
The light reflected by retroreflective articles back toward the source of the light has generally been spread into a cone-like pattern centered on the path the light traveled to the reflector; such a spreading has been necessary so that, for example, light from the headlamps of an oncoming vehicle, reflected back toward the vehicle by a retroreflective sign, will diverge sufficiently to reach the eyes of the driver, who is positioned off-axis from the headlamp beam. In conventional cube-corner retroreflective articles, this cone-like spreading of retroreflected light is obtained through imperfections in the cube-corner retroreflective elements (e.g., non-flatness of the faces, unintended tilting of the faces from the mutually perpendicular positions, etc.) and through diffraction caused by retroreflected light exiting through an aperture defined by the base edges of the three reflecting faces (see Stamm U.S. Pat. No. 3,712,706).
However, the spreading of light from cube-corner retroreflective articles has deficiencies: the cone of retroreflected light is often too narrow for many uses that require reflected light to be seen farther off-axis; and the three-sided nature of the cube-corner retroreflective elements gives the retroreflected cone of light an undesirable asymmetric shape, with the result that cube-corner retroreflective articles suffer from a variation in retroreflective brightness when viewed from different presentation angles (a glossary of terms is at the end of the specification). These deficiencies can be sufficiently severe that two persons sitting side-by-side in a vehicle passing a sign covered with cube-corner retroreflective sheeting may have distinctly different perceptions as to the brightness of the sign.
Tanaka, U.S. Pat. No. 3,817,596 increases the divergence or spreading of light rays from a cube-corner retroreflective article by deliberately tilting the faces of the cube-corner retroreflective elements out of perpendicularity or orthogonality. As taught in papers such as P. R. Yoder, "Study of Light Deviation Errors in Triple Mirrors and Tetrahedral Prisms," Journal of the Optical Society of America, Vol. 48, No. 7, July, 1958; N. E. Rityn, "Optics of Corner Cube Reflectors," Soviet Journal of Optical Technology, Vol. 34, p. 198 et seq and H. D. Eckhardt, "Simple Model of Corner Reflector L Phenomena", Applied Optics, Vol. 10, No. 7, July, 1971, such a tilting of the faces results in light reflected by the cube-corner retroreflective element being divided into as many as six different beams that diverge away from the reference axis of the element and thereby spread the light through a broader range of angles.
Although the spreading of light taught in U.S. Pat. No. 3,817,596 increases the observation angles from which the article may be seen by retroreflection, no effort is made to avoid the basic asymmetry that arises from the three-sided nature of a cube-corner retroreflective element. Further, the spreading reduces retroreflective brightness at commonly experienced smaller observation angles, i.e., the narrow angles near the reference axis, because the light that would ordinarily have been directed to such smaller observation angles is spread through an enlarged region of space. Much of the spread light is wasted, since the article will generally not be viewed from points throughout the enlarged space, and this lost light leaves the retroreflective brightness of the article significantly reduced (see FIG. 6 of U.S. Pat. No. 3,817,596).
Heenan, U.S. Pat. No. 3,833,285, changes the divergence or spreading of light from a cube-corner retroreflective article in a different manner, specifically by incorporating into the article a set of special cube-corner retroreflective elements arranged in a row.
A disadvantage of a retroreflective article as taught in U.S. Pat. No. 3,833,285 is the fact that, in many retroreflective articles, such as traffic control signs, it could be distracting to have a single isolated row of retroreflective elements that distributes light in patterns that are noticeably different from those of other retroreflective elements of the article. For example, instead of seeing a uniformly lit retroreflective sign, an observer would see variations in brightness that could distract from an understanding of the information carried on the sign. Further, a product as described in U.S. Pat. No. 3,833,285 required the precise manufacture of individual pins that are subsequently bundled together to form the row of elements, and it is difficult to precisely form and group together such distinct pins to obtain retroreflection within desired tolerances.
A need exists for an improved retroreflective material which may be individually tailored so as to distribute light retroreflected by the material into a desired pattern or divergence profile and which is efficient at high angles of incidence.